incredulous

paranoid (also paranoidal)

critical, puzzled, quizzical

careful, cautious, guarded, gun-shy, leery (also leary), wary, watchful

cynical, experienced, knowing, sophisticated, worldly, worldly-wise

curious, inquiring, inquisitive, nosy (or nosey), snoopy

uncertain, unconvinced, undecided, undetermined, unsettled, unsure

hesitant

debatable, disputable, doubtable, doubtful, dubious, dubitable, far-fetched, fishy, flimsy, questionable, shaky, suspect, suspicious, unlikely, unreasonable

hopeless, impossible

absurd, comical, farcical, laughable, ludicrous, outlandish, preposterous, ridiculous, risible, silly

indefensible, insupportable, untenable